1.  cell MEMBRANE  The membrane enclosing cytoplasm of a cell is known as Plasma lemma or plasma membrane.  Plasma membrane is composed of lipids and proteins.  The average composition of plasma lemma is as follows:  protein 40%  lipid 60%  It consists lipids, such as, cholesterol, cerebroside and gangleoside, as well as some polysacchrides.

2.  FLUID MOSAIC MODEL  The flouid-mosaic model, a refinement of a lipid- protein bilayer model, acccount for the various properties of biological memranes.  According to this model:-  Two layers of lipid molecules arranged in the centre of a unit membrane, each layer being only one molecule thick.  The polar ends of lipid molecules in a layer are oriented towards the outside, while their nonpolar ends are situated towards the centre of membrane.

3. FLUID MOSAIC MODEL  The protein molecule are embedded either fully or partially into the lipid bilayer.  The membrane-bound proteins are usually complexed with carbohydrates, i.e.,they are glycoproteins.  Many of the proteins receive signals from the outer environment and communicate them into cell’s interior, such proteins are called receptors.

4. Macromolecules found in Membranes Lipids Phospholipid bilayer Cholesterol Proteins Transport proteins Receptor proteins Recognition proteins Carbohydrates Oligosaccharides on glycoproteins

5. The plasma membrane “mosaic” is a collection of proteins.

6. Functions of Membrane Macromolecules Lipids Phospholipid bilayer Forms boundary to isolate cell contents from environment Cholesterol Increases bilayer strength, flexibility Reduces membrane fluidity

7. Functions of Membrane Macromolecules Proteins Transport proteins Regulate movement of water soluble substances Channel proteins have pores that allow passage of ions and small water-soluble molecules Carrier proteins bind to molecules and change shape for delivery across membrane

8. Functions of Membrane Macromolecules Proteins Receptor proteins Docking site for molecules outside the cell Trigger internal cellular response Recognition proteins Identification tags Oligosaccharides aid in cell-cell recognition Cell-surface attachment sites

9. FUNCTIONS OF CELL MEMBRANE  Plasma membrane forms a protective barrier between the intracellular and extracellular environment.  Its chief function is to regulate the movement of various molecules into and out of the cytoplasm.

10. Transport Processes Movement of substances across the cell membrane Passive transport Substances move from [high]  [low] No energy input required Simple Diffusion, Facilitated Diffusion, Osmosis Active transport Substances move from [low]  [high] Requires energy input Protein carriers, Endocytosis, Exocytosis